Recent satellite images suggest North Korea may be about to restart the nuclear reactor seen as its main source of weapons-grade plutonium, a US think-tank said Thursday.

When fully operational, the five-megawatt reactor at the Yongbyon nuclear complex is capable of producing around six kilos (13 pounds) of plutonium a year -- enough for one nuclear bomb, experts say.

North Korea has conducted three nuclear tests, and recently threatened a fourth, amid tensions over fresh US sanctions and UN moves to censure Pyongyang for its human rights record.

The US-Korea Institute at Johns Hopkins University said the latest satellite images showed fresh activity at Yongbyon suggestive of "the early stages of an effort to restart the reactor".

Signs included steam coming from a probable pressure relief valve on a steam pipe just before it enters the turbine building, and meltwater running off the centre of the turbine-building roof, the institute said in a post on its closely-followed 38 North website.

The analysis stressed that the activity was too "limited" to enable an accurate assessment of when the reactor might be tested and become operational.

North Korea shut down the Yongbyon reactor in 2007 under an aid-for-disarmament accord, but began renovating it after its last nuclear test in 2013.

The UN's nuclear watchdog, the International Atomic Energy Agency, said in September last year that steam discharges suggested the reactor might already be operational again.

But analysis of later satellite imagery led other experts to conclude it had either failed to restart properly or had been shut down again -- possibly for refuelling or renovation.

Pictures have also detected activity at a separate Yongbyon facility, thought to be used for enriching uranium.

The North says the purpose is to produce low-enriched uranium for a new reactor it is constructing, but experts suspect that the real goal is weapons-grade uranium, an alternative to plutonium for a nuclear bomb.

Earlier this month, North Korea offered to suspend nuclear tests in exchange for a temporary freeze on US-South Korea joint military exercises.

Washington rejected the offer, which it described as an "implicit threat" to detonate a fourth nuclear device.

A separate analysis on the 38 North website two weeks ago said satellite images of the North's Punggye-ri nuclear test site showed there were no signs that a detonation could be imminent.